Speech quality has become a highly competitive factor in marketing telephony systems. Line or electrical echo, a phenomenon typically caused by imperfect impedance matching of network transmission sections, may significantly degrade the overall speech quality of telephony systems.
Similarly, acoustic echo may also degrade the quality of speech in a telephony system. Acoustic echo may be seen, for example, in communication devices having a near end microphone exposed to a loudspeaker driven by a far end signal or a secondary audio signal. In a full-duplex system, simultaneous two-way communication is enabled where the local user can speak and listen to received speech simultaneously with the remote user. Such simultaneous conversation, however, may create acoustic feedback problems which occur when the near end microphone picks up the far end loudspeaker signal and directs the far end loudspeaker signal back to the remote end. As a result, the remote party may hear a delayed version of their own speech referred to as an acoustic echo.
Echo cancellers have been used to remove far end electrical and acoustic echo. Typically, echo cancellers utilize adaptive filters that model the electro/acoustical echo path. The algorithm coefficients of the filter are continuously adapted to represent the impulse response of the acoustic echo path, such as for example, between the loudspeaker and microphone or the impulse response between the transmit channel and the receive channel of the network interface. The modeled responses are then subtracted from an outgoing communication signal to yield an echo reduced communication signal. However, near end speech may act as an unwanted noise signal causing the adaptive filter to diverge. Therefore, echo cancellers typically include double talk detection logic that halts filter adaptation when near end speech is active.
Acoustic echo cancellers and electrical echo cancellers operating in the presence of a secondary audio tone, such as for example, a pulse metering tone, typically utilize multiple adaptive filters. In the case of electrical echo, separate adaptive filters are typically used to separately cancel the primary and secondary audio signals. Similarly, acoustic echo cancellers typically utilize an adaptive filter to estimate the impulse response between the microphone and loudspeaker. A second electrical echo canceller is typically implemented across the transmit and receive channels to cancel the electric reflection of signals generated by an impedance mismatch at the hybrid interface.
However, the utilization of multiple adaptive filters increases the complexity and computational intensity of the echo canceller system. In addition, double talk detection logic typically declares near end speech active whenever a local near end signal is present in either adaptive filter. Therefore, echo canceller performance may be seriously degraded in applications where a secondary audio tone or signal is present at substantially all times.
Therefore it would be advantageous to have a system and method that enables filter adaptation, and thus echo cancellation in the presence of a known secondary audio signal such as a music signal, a pulse metering tone or the sound of a computer game.